1. Field of the Invention
This invention relates to hole drilling and more particularly to a method of using laser radiation to form holes free of recast layers and heat affected zones.
2. Description of the Prior Art
The use of laser radiation as long pulses having durations varying from microseconds to milliseconds, pulse energies of at least one joule and repetition rates less than ten pulses per second is well known to be capable of efficiently removing material from a workpiece to produce a hole. The energy of the pulse incident on the workpiece is partially utilized to produce vaporization of surface material in a time interval short compared to the pulse duration, while the remaining energy of the pulse is absorbed to produce melting of subsurface material. The surface vaporization results in a cooling mechanism which reduces the surface temperature and causes the maximum temperature area to lie below the surface. The subsurface melting generates a high subsurface pressure which produces an explosive expulsion of the molten material from the workpiece. As the melted material is expelled from the workpiece, portions of the melt solidifies along the walls of the hole producing a recast layer. The material structure of the recast layer is dissimilar to the base material, is usually of low ductility and is prone to shrinkage cracks which are capable of propagating into the base material. Also, the subsurface melting results in significant heat conduction into the base material of the workpiece resulting in the formation of a heat affected zone within the wall of the hole.
Deep holes are usually obtained by repetitively interacting the pulses of laser radiation with the workpiece. The hole obtained is typically noncircular, has a recast layer having variations in its thickness resulting in variations in the diameter of the hole along its length and generally has significant taper from end to end which is undesirable for most applications.
The explosive expulsion of melted material results in molten splatter being ejected from the workpiece in a direction along the path of the laser radiation. Unless protection is provided, the splatter usually deposits on the focusing optics causing distortion of the pulse shape and absorption of the pulse energy, which reduces the efficiency of the material removal process. The optics are typically protected by inserting a transparent protective shield between the optics and the workpiece to collect the splatter or by the utilization of a stream of gas across the path of the laser radiation to deflect the splatter and inhibit impingement on the optics.
The use of laser radiation as pulses having durations of at least twenty nanoseconds, energies less than one hundred millijoules and repetition rates less than ten pulses per second are also known to remove material from a workpiece. The material removal process is comparable to that of the long pulses and holes are obtained having recast layers and heat affected zones.